The compressive load carrying capacity of a layer of resilient material may be increased several hundred percent by incorporating a plurality of spaced, parallel laminae fabricated of nonextensible material and oriented generally perpendicular to the direction of the anticipated compressive load. The laminae increase the compressive load carrying capacity of the resilient material by reducing the ability of the material to deflect or bulge in directions transverse to the direction of the compressive load. At the same time, the ability of the material to yield in shear or torsion in directions parallel to the laminations or transverse to the direction of the compressive load is substantially unaffected. The characteristics of such a laminated resilient material have resulted in the commercial acceptance for a variety of applications of bearings incorporating the material. One area of particular importance is the mounting of helicopter rotor blades on an associated rotor hub.
In a typical mounting system for the blades of a helicopter rotor, as shown for example in Rybicki et al. U.S. Pat. No. 3,829,239, each rotor blade is secured to a rotor hub by two, serially arranged laminated bearings. One laminated bearing, which has annular, disc-shaped laminations, resists the centrifugal load on the rotor blade and permits oscillatory pitch-change movements of the blade about its longitudinal axis. The other bearing, which has annular, spherically-shaped laminations, also resists the centrifugal load on the rotor blade and accommodates pitch-change, flapping and lead-lag movements of the blade. Since the pitch-change rotations of the blade are of a relatively large magnitude (i.e. 10.degree. to 15.degree. in each rotational direction), particularly when compared to the lead-lag and flapping motions, the combined torsional movement capabilities of the two laminated bearings must be relatively large. For economic and space reasons, the bearing having disc-shaped laminations is conventionally designed to accommodate a greater proportion of the torsional motion. As a result, the disc-type or thrust bearing is relatively long or tall. With increasing length or height, however, the bearing becomes increasingly unstable in a lateral or radial direction.
The lateral instability associated with a tall or long laminated bearing has been recognized and various proposals have been made to counter the instability. One such proposal is to modify the disc-like configuration of the laminations in the bearing so as to resist the lateral movements of individual laminations which result in buckling of the bearing. Representative configurations providing lateral stability are described and illustrated in Hinks et al. U.S. Pat. No. 3,083,065 and Peterson U.S. Pat. No. 3,292,711. Bearings having laminations configured to resemble the laminations of the bearing shown in FIGS. 2 and 6 of the Peterson U.S. Pat. No. 3,292,711 have been utilized successfully in helicopter rotor blade retention systems. Nonetheless, as technology in the construction of helicopters has advanced, increased demands, in terms of higher centrifugal loads and prolonged service life, have been made on laminated bearings utilized in helicopters. Laminated bearings having laminations shaped generally as shown in the Peterson U.S. Pat. No. 3,292,711 and more specifically as shown in FIGS. 2 and 3 of Johnson U.S. Pat. No. 3,807,896 and in FIGS. 1(b) and 3 of an article entitled "Elastomeric Bearing Application to Helicopter Tail Rotor Designs", authored by C. H. Fagan and appearing in the Journal of the American Helicopter Society, Volume 13, No. 4 (October 1968), have been unable to satisfy increased service life requirements. Such bearings have characteristically failed through failure and extrusion of the elastomeric laminations adjacent to each end of the bearing.